It is known that in the palladium catalyzed dimerization of low molecular weight aliphatic mono-olefins a catalytic system can be used, which is formed by combining a palladium halide with an organoaluminum halide and preferably with a monodentate phosphine, arsine or stibine (cf. UK Patent specification No. 1,153,519). It is also known to dimerize propene in the presence of a catalytic system formed by combining pentanedionatopalladium with ethylaluminum dichloride and an organic monodentate phosphine or phosphite (cf. Angew. Chem. Int. Ed. 14, 104-105 (1975).
Because of the presence of organoaluminum halides in the catalytic systems the above dimerization reactions should be carried out under strictly anhydrous conditions.
Further, it is known how to dimerize ethene with a catalytic system formed by combining tetrakis(acetonitrile)palladium ditetrafluoroborate with a monodentate phosphine ligand (cf. J. Am. Chem. Soc. 103, 4627-4629 (1981) and 104, 3520-3522 (1982). Once again, this dimerization has to be carried out under strictly anhydrous conditions (cf. Transition metal catalyzed polymerisations; Alkenes and Dienes, Part A, Edited by R. P. Quirk, 1983, pp. 341-354).